Various types of test probes are used in the automatic testing of electrical circuits, depending upon the type of electrical device under test. Some types of probes are particularly adapted to use on printed circuit boards for test of the electrical continuity of the test sites whereas other probes are used in testing wiring harnesses. Switch probes have often been used for testing cable harnesses for vehicles. Switch probes provide electrical continuity only when the switch probe plunger is pushed back into the probe body a sufficient distance to close an internal switch. This produces a closed circuit in the testing probe so that electrical current can flow to the device under test. The flow of electrical current in turn provides a closed circuit in the testing apparatus, indicating that the cable harness terminal has been seated or mounted properly.
In the testing of unmounted cable harnesses, a relatively large outward force is applied to each cable harness terminal by the switch probe during each cycle. If the terminal is not seated properly in the cable harness, the terminal is pushed out of the way by the probe and the switch is prevented from closing. This produces an open circuit in the tester, indicating that the terminal was not seated or mounted properly. If the terminal is mounted properly in the harness, the pressure applied by the switch probe to the terminal during the cycle causes the switch probe plunger to travel sufficiently far to close the switch. This produces a closed circuit on the tester, indicating that the cable harness terminal has been seated or mounted properly.
U.S. Pat. No. 4,983,909 explains switch probes and provides in its FIG. 1 an exemplary prior art switch probe. It is disclosed in the aforementioned patent that a typical switch probe is a miniature size device which includes an outer barrel, a terminal at one end of the barrel, a moveable plunger projecting from the opposite end of the barrel, and a coil spring inside the barrel between the plunger and the terminal. A downward force applied to the plunger against the bias of the spring moves the plunger toward a switch point on the terminal inside the probe. The force applied under test is controlled by the spring constant of the coil spring. If the cable harness is faulty, maximum design force is not reached and the plunger does not close the switch. If the cable harness terminal is wired properly, the maximum design force is reached and the plunger contacts the switch point.
Further as set forth herein, the switch probes are subject to an extreme amount of abuse because of the constant rapid cycling for testing millions of cable harness terminals. Because the terminals of the device under test are closely spaced apart, the switch probes are typically a miniature size and are spaced apart by a distance of about one-tenth of an inch on center.
A continuing problem with switch probes has been that they are typically not designed for use with standard receptacles, but are difficult to remove and replace in the test fixture. The use of a switch probe of standard size to fit in a receptacle would lower cost and provide ease of replacement. Additionally, in some switch probes, the switch probe can be signaled on when the internal switch portions are not fully in physical contact with each other.